Track circuits are used in the railroad industry to detect the presence of a train in a block of track. An AC overlay track circuit includes a transmitter and a receiver, with the transmitter configured to transmit an AC signal through the track rails at one end of a block of track and the receiver connected to the rails at the other end of the block and configured to detect the signal. Other than the connection through the track rails, there is typically no connection between the transmitter and receiver for a block. When a train is present in a block of track monitored by a track circuit, the train shunts, or shorts, the two rails, with the result that no signal is received at the receiver. Thus, the receiver uses the presence or absence of a detected signal to indicate whether or not a train is present in the block. It is therefore very important that a receiver in a particular block of interest not interpret spurious signals or stray signals from a transmitter in another block of track or some other transmitter as originating from the transmitter associated with the block of interest.
Safetran's existing AC overlay track circuit product, the PSO III, had the capability of transmitting on 16 different frequencies. The use of different frequencies was intended to allowed track circuits to operate in close proximity to each other without fear that signals from a transmitter in a first block would be received by a receiver in a second block and be misinterpreted as originating from the transmitter associated with the second block. However, due to certain installations in dense track areas, one of two digital codes (A and C) were further used in order to provide a total of 32 unique combinations of frequencies and codes. The carrier signal is modulated by the code using a FSK technique.
In order to generate one of the 16 different frequencies, the PSO III employed one of 16 different transmitter cards and 16 different receiver cards, with separate versions of each of these cards for the two different addresses. Thus, keeping replacement cards for all of the different frequencies and addresses on hand in order to replace any circuit cards that became defective required stocking a large number of different circuit cards.